The invention has an application particularly, but not exclusively, in relation to the exploration for oil and gas. More specifically, the present invention concerns a casing drilling shoe primarily for use in oil well drilling.
When drilling subterranean formations for the purpose of oil exploration it is normal to firstly drill a section of hole of a particular diameter and then remove the drill bit from the well bore. A tubular member of lesser diameter, known as casing, is placed in the well bore and subsequently the annulus between the drilled hole and the outside of the casing is filled with cement. The purpose of the cement is to isolate certain of the subterranean strata from each other. The next operation is to pass through the casing with a smaller diameter drill bit and drill the further section of hole beyond the previously attained depth. This sequence is repeated as many times as necessary, with smaller and smaller components, until the ultimate desired depth of the well is achieved.
Positioned at the end of each casing string is a rounded guiding component known as a shoe. Typically, the leading edge of the shoe is constructed from cement, to enable it to be easily drilled through by the next drill bit.
The cost of oil exploration particularly in offshore regions is extremely high. For instance, the operating cost of a semi-submersible drill rig is often in excess of $100,000 per day (June 1998). Thus it is in the interest of the operator to minimise the time taken to drill a well. At great depths, the round trip time to pull out a drill bit and replace it with another one can be many hours. This xe2x80x9ctripxe2x80x9d time is seen as non-productive and wasteful, and a significant advantage can be gained, if, having drilled to target depth the drill bit did not have to be removed from the well bore. In this way, a trip could be saved.
A proposed solution would be to attach the drill bit to the leading end of the casing string and drill to target depth and then cement the casing. Certain advances in recent years have rendered this solution more viable, including the provision of premium casing threads able to take the necessary drilling torque, and rotary top drives able to transmit the torque directly to the trailing end of a drill string are commonplace.
However, technical difficulties have not entirely been overcome and this is clearly evidenced by the fact that the industry has not adopted xe2x80x9cdrilling with casingxe2x80x9d to date.
One major remaining issue concerns the drill bit itself. By design drill bits are robust devices able to withstand the rigours of a downhole environment. They are generally made from hard materials such as steel or tungsten carbide matrix. After cementing the drilled-in casing the subsequent drill bit would have to pass through the previous one before exiting the end of the casing string. Unfortunately, modern drill bits optimised for rock removal are unable to drill through the materials from which they themselves are constructed without sustaining a level of damage which would render the task of drilling the next section of rock formation impossible. It is possible to drill through a drill bit with special tools known as mills, but these tools are unable to penetrate rock formations effectively and so the mill would have to be xe2x80x9ctrippedxe2x80x9d from the hole and replaced with a drill bit. In this case, the trip saving advantage gained by drilling with casing would have been lost.
Thus it is recognised in the present invention that considerable advantage is to be gained in the provision of a casing shoe that is able to drill rock formations effectively, but which itself is capable of being drilled by standard oilfield drill bits.
Drilling shoes have been available in the past specifically for attachment to casing, although usually for special applications such as a situation where the lowermost rock strata of a section of a well to be drilled are extremely unconsolidated and there is a consequential risk that after the drill bit is removed from the well the rock strata may collapse into the well bore. This then renders the process of placing the casing in the well bore difficult or impossible. Such casing shoes have invariably been made from the hard materials associated with normal drill bits and as such cannot be drilled through.
Also, casing whilst drilling systems have been and continue to be available to the industry. One such system involves running a casing string and a drill string in tandem. Attached to the leading end of the casing string is a core type bit able to cut a xe2x80x9ckerfxe2x80x9d of formation. Positioned at the leading end of the drill string is a drill bit driven by a hydraulic motor. Thus, the core bit and the drill bit together can drill a hole of the required diameter. Prior to performing the cementing operation however, the drill bit has to be removed from the well bore and thus the expensive trip is not saved.
Probably the apparatus which comes closest to overcoming the afore-described problems is known as a reamer shoe. Reamer shoes have become available over the last few years and are devices that are able to drill over the extreme outer diameter of the tool but which have an inner section manufactured from a material which is drillable with drill bits. The objective or utility of these tools, however, is to help the casing string enter a difficult well bore and when landed and cemented, pose no obstruction to the subsequent drill bit.
According to the present invention there is provided a casing drilling shoe adapted for attachment to a casing string, wherein the shoe comprises an outer drilling section constructed of a relatively hard material and an inner section constructed of a readily drillable material, and wherein means is provided for controllably displacing the outer drilling section to enable the shoe to be drilled through using a standard drill bit and subsequently penetrated by a reduced diameter casing string or liner.
Optionally, the outer section may be made of steel and the inner section may be made of aluminium.
Preferably, the outer section is provided with one or more blades, wherein the blades are moveable from a first or drilling position to a second or displaced position. Preferably, when the blades are in the first or drilling position they extend in a lateral or radial direction to such extent as to allow for drilling to be performed over the full face of the shoe. This enables the casing shoe to progress beyond the furthest point previously attained in a particular well.
The means for displacing the outer drilling section may comprise of a means for imparting a downward thrust on the inner section sufficient to cause the inner section to move in a down-hole direction relative to the outer drilling section. The means may include an obturating member for obstructing the flow of drilling mud so as to enable increased pressure to be obtained above the inner section, the pressure being adapted to impart the downward thrust.
Typically, the direction of displacement of the outer section has a radial component.
Also according to the invention there is provided a casing drilling shoe adapted for attachment to a casing string, wherein the shoe comprises an outer drilling section constructed of a relatively hard material and an inner section constructed of a readily drillable material, and wherein means is provided for controllably displacing the outer drilling section to a position whereby it does not interfere with subsequent drilling through the shoe for the placement of further casing or a liner down-hole.